1. Field of the Invention
The present invention relates to a manufacturing method for an integrated resonator wherein a piezo-electric thin film resonator is formed integrated together with other functional devices on a silicon monocrystal substrate.
2. Description of the Prior Art
In a conventional bulk wave resonator which utilizes the primary thickness vibration of a piezo-electric substrate such as a ceramic substrate or a crystal substrate, the applicable resonance frequency is limited to a frequency of order of several tens MHz since this type of the substrate is impossible to make the thickness thereof thinner than several tens .mu.m because of restrictions in the processing technique and/or the mechanical strength thereof.
In order to avoid this limitation, the piezo-electric thin film resonator of so called diaphragm type has been developed which is able to manufacture with use of the production technique for semiconductor and can actuate in an ultrahigh frequency zone such as VHF or UHF.
As shown in FIGS. 4 and 5, in a conventional production method for the diaphragm type of piezo-electric resonator, at first, a P.sup.+ silicon layer 2 is formed on a surface of a silicon monocrystal substrate 1 and an oxidized silicon thin film 3 is formed on the P.sup.+ silicon layer 2. On the oxidized silicon thin film 3, first electrode 4, a piezo-electric thin film 5 of ZnO and a second electrode 6 are formed in a stacked state. Thereafter, the substrate 1 is etched anisotropically from the other surface thereof to form a hole 7 for allowing the thickness vibration of the piezo-electric thin film 5.
However, this manufacturing method has various disadvantages as follows;
First of all, it is difficult to integrate the above piezo-electric thin film resonator on the silicon monocrystal substrate 1 together with other functional devices. Namely, this method needs an extraordinal process for forming the hole from the bottom surface of the substrate which is not included in the ordinal production method for an integrated circuit and, therefore, the existing production line equipment for the semiconductor is not available for the manufacture of this type of piezo-electric thin film resonator.
Further, it is difficult to control the thickness of the P.sup.+ silicon layer 2 on which the piezo-electric thin film 5 is to be formed; devices formed on the substrate are damaged in the etching process which needs a relatively long time; the size of this type piezo-electric resonator becomes inevitably large since an area necessary for etching is impossible to minimize because of the anisotropic etching; the other surface of the substrate cannot be utilized for forming devices; and it becomes necessary to set the etching area so as to correspond to the piezo-electric resonator with use of the photo-lithographic technique.
In order to solve these problems, there is proposed a piezo-electric thin film resonator of ZnO having a structure as shown in FIG. 6.
Upon production thereof, an SiO.sub.2 layer 12, insulating layers 13 and 14 and a passivation layer are formed on a silicon substrate 11 in a stacked state. On the passivation layer 15, a thin film 16 of ZnO is formed as a dummy layer to be ethched. Further, an SiO.sub.2 layer 17, a ZnO layer 18 and an SiO.sub.2 layer 19 are stacked on the ZnO thin film 16. Thereafter, the ZnO thin film 16 is etched from a side thereof with use of hydrochloric acid to form a gap 21. Further, Au/Ti electrodes 22 and 23 are formed on both of surfaces of the ZnO layer 18. Thus, a ZnO piezo-electric resonator of diaphragm type is formed above the gap 21.
However, in this structure, the height of the resonator becomes higher than those of other devices to be integrated on the substrate since it is formed above the gap 21. This invites differences in the height between the resonator and other devices, which causes bad affections upon forming other functional devices by preventing a mask from contacting to areas for other devices to be formed.